Many application circuits utilize amplifiers to achieve desired functionality. Analog-to-digital converters (ADCs), for example, typically require amplification during the conversion of an analog signal to a digital signal. Application circuits may also utilize an amplifier (e.g., an operational amplifier) arranged in a feedback configuration to realize a predetermined gain as a function of frequency. FIG. 1A depicts an exemplary of an operational amplifier 10 in a feedback configuration. A ΔΣ ADC, e.g., typically utilizes a feedback amplifier, having a specified predetermined frequency response, to achieve performance goals such as noise shaping. Other application circuits, such as a tuner or a general amplifier, can also require amplification having a predetermined frequency response. An operational amplifier typically needs to be at least conditionally stable, as arranged in a feedback configuration, to achieve a predetermined frequency response.
One operational-amplifier architecture that can be used includes a two-stage operational amplifier having Miller compensation. However, such a two-stage operational amplifier may not provide enough loop gain in low frequencies. Moreover, extending the Miller-compensated operational amplifier architecture structure to more than two stages can typically only achieve about −6 dB/octave of loop gain at frequencies leading up to the unity gain frequency.
An operational-amplifier architecture that can be used to provide higher loop gain includes embodiments of multi-path, multi-stage feed-forward operational amplifier circuits. FIG. 1B depicts an exemplary embodiment of a multi-path, multi-stage feed-forward operational amplifier circuit 20. The depicted amplifier circuit 20 includes a plurality of distinct amplification paths 24, each amplification path 24 including a number of amplifiers and capacitors and being considered to be of a certain order based on its number of amplifiers. A first-order amplification path 24a includes a single amplifier 28a, a second-order amplification path 24b includes two amplifiers 28b, 28c (along with a capacitor C1a), a third-order amplification path 24c contains three amplifiers 28d, 28e, 28f (along with capacitors C1b, C1c), and a fourth-order amplification path 24d includes four amplifiers 28g, 28h, 28i, 28j (along with capacitors C1d, C1e, C1f). The amplifiers are depicted as being transconductance amplifiers (transconductors), i.e., voltage-to-current amplifiers. However, it is also possible to use voltage-to-voltage amplifiers or current-to-current amplifiers. The depicted signal paths can be either single-ended or differential signal paths.
One advantage of a multi-path, multi-stage feed-forward amplifier architecture is that it typically provides a steeper loop gain below unity-gain frequency, e.g., greater than −6 dB/octave, than does a multi-stage Miller-compensated architecture. Moreover, each amplification path can be individually tailored to contribute a different frequency response. Relatively lower-order amplification paths can typically be used to implement a lower DC-gain, higher-bandwidth frequency response, whereas higher-order amplification paths can typically be used to implement a higher DC-gain, lower-bandwidth frequency response. The selective combination of such different frequency responses can typically implement much higher loop gain at low frequencies than Miller-compensated operational amplifier architectures, and provide improved closed-loop accuracy and lower distortion. However, a steeper loop gain can also mean that the operational amplifier is only conditionally stable, i.e., stable only within a certain range of feedback factors.
One problem with the embodiment of the multi-path, multi-stage amplifier circuit 20 depicted in FIG. 1B, however, is that it can be regarded as inefficient from the context of both size and power. The large number of independent amplifiers 28a-28j requires both a large chip area to implement and a large power to operate. Thus, there exists a need for a multi-path, multi-stage amplifier circuit which is more efficient in size and power use, while also achieving good noise and output-current driving performance.